This invention relates to molding of synthetic resin to produce features that are integral with a base, and has particular application to the production of fastener elements for touch fasteners and the like.
Hook elements for hook-and-loop touch fasteners and other products are effectively produced by the machine and method of Fischer U.S. Pat. No. 4,794,028. In commercial production, a mold roll is formed by a large number of thin, disk-shaped mold rings (sometimes called mold plates) and spacer rings which are stacked concentrically about a central barrel.
Discrete products are also injection molded with fastener elements extending from a base surface by employing, as part of the mold, a series of stacked mold plates defining an array of mold cavities.
At the periphery of the mold rings or mold plates are cavities for molding the hook elements. In current production machines each cavity of a mold ring has been formed, one at a time, by wire electro-discharge machining (EDM). In the wire EDM process, an electrical discharge between a wire and the plate removes material from the plate while the wire is moved along a specified path to cut a profile through the mold plate. The minimum radius arc that can be cut is determined by the radius of the EDM wire.
Molten resin is forced into the mold cavities, tending to raise the temperature of the mold rings. In practice of the Fischer method, a fluid coolant is circulated through cooling passages within the barrel on which the rings are mounted to remove the heat from the rings. In this way an appropriate temperature of the mold cavities is maintained so that the product becomes sufficiently solid that it can be withdrawn on a continuous basis, typically without opening the mold cavities.
Given the large capital costs of the equipment and the need to form improved products, it is desirable to find improvements for implementing the Fischer machine and for molding very small fastener elements.
According to one aspect of the invention it has been realized that because the cooling device has been a separate member from the mold rings, the speed with which heat can be removed is detrimentally affected by resistance to heat transfer at the interface between rings and cooling device. We realize that even in areas in which direct contact between the rings and the cooling device may occur, the resistance to heat transfer caused, e.g. by microscopic surface imperfections, can adversely limit operation of the process. Furthermore, we realize that manufacturing tolerances of the rings and cooling device, and the manner in which they are assembled, result in small nonuniform air gaps between various portions of the ring and cooling device surfaces. These act as detrimental thermal insulators that produce non-uniformities.
We have realized that nonuniform cooling, non-uniformities in the product produced, limitations on the speed of operation, and other drawbacks can be overcome by eliminating interfaces between the cooling medium and the rings of the mold roll.
According to an aspect of the invention, fluid cooling passages are defined in the mold rings themselves and means are provided to prevent leakage of the cooling fluid between the individual rings.
The invention enables the temperature of the mold cavities to be maintained more uniformly around and along the mold roll even at high speed operation, thereby achieving advantages in product quality and throughput.
According to a further aspect of the invention, the means to prevent leakage at the mating rings comprises an array of axially extending tie rods that apply significant compressive force between the rings of the stack. Besides creating a sealing effect between the faces of mating rings at the aligned coolant passages, this axial compression is found to be important in improving the bending resistance of the assembled roll. This enables uniform and tightly controlled thickness of the base layer of the product to be achieved. In turn, this enables production of thinner base layers. This can lower product cost and achieve highly flexible products that are useful, for example, on curved or flexing surfaces. The improved stiffness of the mold roll further enables the use of longer mold rolls and improved machine geometries for producing wider products.
According to one aspect of the invention, a molding apparatus useful for continuously forming features of synthetic resin integral with a base has a shaft and a mold roll having an axis and comprising a multiplicity of thin, sheet-form rings of heat-conducting material. The rings each have an inner diameter, an outer diameter, and a substantially circular array of coolant holes. The rings are mounted to form a stack about the axis of the mold roll. At least many of the rings are mold rings, each mold ring having a series of mold cavities disposed at its periphery.
The mold roll features means for axially compressing the stack and an array of fluid passages for liquid coolant. The passages are formed by the aligned coolant holes of each ring, and extend through the roll for cooling the mold cavities via heat transfer from the material of the disks substantially directly to liquid coolant in contact with the edges of the disk material about the holes.
The apparatus also includes means for introducing heated resin to the surface of the mold roll under pressure conditions, filling the mold cavities and forming a base layer integral with features molded in the cavities. Also included in the apparatus is a means for removing the resultant product from the mold roll after the product has cooled to a desired temperature below the temperature of introduction of the resin.
In a preferred embodiment, the apparatus further includes an inlet manifold mounted at a first end of the mold roll for directing cooling liquid into the coolant passages through the rings.
In another embodiment, a return passage communicates with the coolant passages through the rings and extends axially through the roll. Preferably, the return passage is in a shaft upon which the rings are mounted. In a current configuration, a large number of the coolant passages are arranged in a circle adjacent the periphery of the mold roll, preferably about 50 cooling passages disposed within about one-half inch of the periphery of a mold roll with a diameter between about 8 to 12 inches.
In preferred embodiments, the cavities are shaped to form fastener elements for touch fasteners. Preferably, the mold cavities are hook-shaped to form fastener hooks.
In one embodiment the cavities and the holes are of photochemical etched form. In another embodiment, the cavities and the holes are of laser machined form.
In some embodiments, the means for compressing the stack comprises a circumferential array of tie-rods extending through the rings parallel to the shaft and exerting axial compressive force on the aligned rings. In a presently preferred configuration, there are at least 6 tie rods disposed within about 2 inches of the periphery of a mold roll with a diameter of the order of 8 to 12 inches.
Some embodiments include means to maintain subatmospheric pressure on cooling fluid in the passages.
In some embodiments a sealant material is employed in the vicinity of the fluid holes to promote sealing. In some cases the rings are coated on their sides adjacent the fluid holes with sealant material. In some cases the sealant material is fluid-deposited in interstices between the rings. The sealant material is preferably hydrophobic.
In some embodiments the cavities do not extend through the thickness of the ring.
In some configurations the cavities are on a first side of a mold ring. In other configurations the cavities are on both sides of a mold ring.
In some situations the cavities advantageously each have an enclosing face which is substantially concave.
In some embodiments the fluid holes, the inner diameter and the outer diameter of the mold ring are of photochemical etched form extending through the thickness of the ring.
In the presently preferred embodiment the apparatus also has a pressure roll positioned in proximity to the mold roll to form at least a broad surface of the base. The pressure roll has a circular array of passages for liquid coolant, with the passages extending through the roll for cooling the periphery of the pressure roll via heat transfer from the material of the roll substantially directly to liquid coolant in contact with the inner surface of the passages.
In some embodiments the pressure roll also has mold cavities disposed at its periphery.
According to another aspect of the invention, a method of forming a mold roll for forming fastener elements for touch fasteners is provided. The mold roll comprises a stack of thin, sheet-form mold rings of heat-conducting material having an inner diameter and an outer diameter, each ring having a circular array of holes near its periphery. At least some rings are mold rings, each having a series of mold cavities disposed at its periphery. The method includes forming the cavities and holes, and stacking the rings in alignment such that the aligned holes form coolant passages through the mold roll. The passages extend through the roll for cooling the mold cavities via heat transfer from the material of the disks substantially directly to liquid coolant in contact with the edges of the disk material about the holes.
In some embodiments, forming the cavities and holes is performed by a photochemical etching process. In some other embodiments, forming the cavities and holes is performed by a laser machining process.
According to another aspect of the invention, a method of molding an article comprised of synthetic resin includes providing the apparatus described above, filling the mold cavities with heated resin under pressure conditions while passing cooling liquid through the cooling passages, and removing the resultant product from the mold roll after the product has cooled to a desired temperature below the temperature of introduction of the resin.
According to another aspect of the invention, a method of aligning a multiplicity of thin, disk-shaped mold rings, each having an array of aligning holes and an outer circumferential surface, to form a mold roll, is provided. The method includes
1. providing an alignment shell defining a circular aligning surface, and at least one aligning bar;
2. stacking the rings together to form a stack, each ring being supported on its outer circumferential surface by the aligning surface, with the aligning bar extending through a the aligning hole in each ring;
3. axially compressing the stack of rings to maintain the radial alignment provided by the circular aligning surface; and
4. removing the aligned stack of rings from the alignment shell.
We have also realized that cavities for molding fastener elements, especially fastener elements with small features (e.g., hooks with sharp tips), may be formed by laser cutting methods. According to another aspect of the invention, a method of making a mold with fastener element-shaped cavities is provided. The method includes forming mold plates from flat plate stock by laser-cutting each plate to form an array of mold surfaces that extend inwardly from an edge of the mold plate for defining respective fastener element-shaped cavities.
Certain preferred embodiments of the method of the invention include: stacking the mold plates with their edges aligned to form a surface of a mold having rows of fastener-shaped mold cavities; laser-cutting coolant holes through the thickness of the plates at locations spaced from their edges; preferably, aligning the holes during the stacking to form coolant passages.
In some cases the mold includes round mold plates with laser-cut outer edges; preferably, the mold plates are axially stacked to form a mold roll. In some other cases, the mold defines a closeable and openable mold cavity for injection-molding of part.
In certain preferred embodiments, the mold surfaces define fastener element features having end radii of less than about 0.001 inch; preferably, less than about 0.0005 inch.
In a particularly preferred embodiment, at some of the fastener-shaped mold cavities are formed by the mold surfaces of two or more adjacent, stacked mold plates.
In certain embodiments, the laser-cutting is controlled to produce mold surfaces extending through only a portion of the thickness of their respective mold plates; preferably, the mold surfaces are concave. In other embodiments, at least some of the mold surfaces extend through their respective mold plates; preferably, these surfaces are generally parallel to the surface of the mold.
For greater efficiency, the mold surfaces of a plurality of mold plates can be simultaneously formed by laser-cutting.
In some cases, forming the mold plates include, after laser-cutting the mold surfaces, smoothing the mold surfaces, such as immersion in a chemical etchant.
According to another aspect, the invention provides a mold plate produced by laser-cutting, having any number of the features heretofore described with respect to the method of the invention.
The invention may provide one or more advantages. For example, the invention is particularly suitable for forming small fastener elements (e.g., elements having an overall height smaller than 0.020 inch) with extremely small tip radii (e.g., less than 0.001 inch). The outer edge of a mold plate and the mold surfaces around the edge can be simultaneously formed. Mold plates can thus be cost-effectively produced with accurately-formed mold surfaces.